A number of optically-based techniques have been used historically to detect fires and explosions. These techniques include ultraviolet detection, ultraviolet/infrared detection, infrared detection, and ultraviolet/visible/infrared detection. The comparative desirability of using one technique over another depends upon many parameters, including the nature of the physical environment where the fire detector is to be used, the types of fuels involved, and the number and type of false alarm radiant sources to which the fire detection system must not respond. The present invention relates to infrared optical fire and explosion detection systems where the optical sensors involved in the detection system are restricted to sensing radiation within the infrared spectrum.
The following patents and patent publications describe fire detection systems/fire detectors that detect the presence of a fire and/or an exploding ammunition round by sensing IR radiation emissions; U.S. Pat. No. 3,825,754, U.S. Pat. No. 3,859,520, U.S. Pat. No. 3,931,521, U.S. Pat. No. 4,296,324, U.S. Pat. No. 4,357,534, U.S. Pat. No. 4,373,136, U.S. Pat. No. 4,414,542, U.S. Pat. No. 4,415,806, U.S. Pat. No. 4,421,984, U.S. Pat. No. 4,423,326, U.S. Pat. No. 4,497,373, U.S. Pat. No. 4,533,834, U.S. Pat. No. 5,051,590, U.S. Pat. No. 5,051,595, U.S. Pat. No. 5,064,271, U.S. Pat. No. 5,162,658, Canadian Pat. No. 1,211,183, EPO Applic. No. 175,032, U.K. Applic. No. 2,103,789, U.K. Applic. No. 2,126,713, and U.K. Applic. No. 2,142,757. The described systems/detectors differ in the number of IR spectral regions being sensed, the bandwidths and bandwidth centers for the IR spectral regions being sensed, and the manner in which the detector outputs are processed and evaluated to detect the presence of a fire.
None of these systems/detectors describe a system that simultaneously senses IR radiation in two IR spectral regions so as to be capable of detecting fires fueled by either hydrocarbons or non-hydrocarbons. Also not described are systems having two detectors that are filtered to simultaneously sense IR radiation in five distinct and separate IR spectral regions. Further, none of the above describe a reference channel detector that simultaneously senses IR radiation in three distinct and separate IR spectral regions, in particular three spectral regions whose bandwidths are selected so as to be responsive as possible to non-fire radiation sources but non-responsive to the IR radiation being sensed by a fire channel detector (i.e., fire generated IR radiation).
The following indicates how many IR spectral regions are being sensed in the systems/detectors described in the above referenced patents/publications. One (U.S. Pat. No. 4,415,806) describes a system/detector that senses a single IR spectral region; five (U.S. Pat Nos. 3,825,754, 3,859,520, 4,421,984, 5,051,590, 5,051,595) describe a system/detector that senses three IR spectral regions; one (U.S. Pat. No. 4,357,534) describes a system/detector in which two or three IR spectral regions are being sensed; one (U.S. Pat. No. 5,162,658) describes a system/detector in which six IR spectral regions are being sensed; and the remaining fourteen describe a system/detector that senses two IR spectral regions.
Two of the described three channel systems (U.S. Pat Nos. 5,051,590, 5,051,595) have a single fire channel, a single reference channel for sensing IR radiation emissions and a third channel that senses temperature. This system uses separate detectors to sense the IR radiation/temperature.
Another of the three channel described systems (U.S. Pat No. 3,825,754) has a fire channel with two separate detectors to sense two spectral regions and a third channel that is used to sense IR radiation emissions in the bandwidth appropriate to detect an exploding ammunition round. The three channel system described in U.S. Pat No. 4,421,984, is another fire and explosion detection system that is supposed to detect hydrocarbon fires involved with exploding ammunition rounds. This system uses three separate detectors to sense the IR spectral regions of interest. Since the purpose of these systems is to detect hydrocarbon fires in the presence of exploding ammunition rounds, the IR spectral regions selected for reference channel purposes are those appropriate for this particular application.
In U.S. Pat. No. 3,859,520 the fire detection system described uses three discrete IR optical sensors each having an active element. One sensor is filtered to respond to IR radiation near 3.7 microns, the second is filtered to respond to IR radiation near 4.4 microns and the third sensor is filtered to respond to IR radiation near 5.1 microns. Alternatively the filters can be configured so the sensors are responsive to IR radiation near 2.3 microns, near 2.5 to 2.8 microns and near 3.5 microns. In this three channel system, the second sensor or channel (e.g., the one filtered near 4.4 microns) is the fire channel. The signal output level from this channel is compared with the signal outputs from the sensors associated with the first and third channels. Detection of a fire or triggering of the system takes place if the detected intensity from the fire channel sensor exceeds the sum of the intensities for the first and third channels.
As indicated above this system is not capable of detecting fires that are fueled by hydrocarbon and/or non-hydrocarbons, rather the described system is particularly designed to detect a methane-air fire in a coal mine using either sensor configuration. This system uses a separate sensor to detect each of the IR spectral regions that is being sensed.
While the first sensor configuration of the described system is responsive to fires fueled by hydrocarbons, since its fire channel is limited to an IR passband center of 4.4 microns the system will be non-responsive to hydrogen (i.e., non-hydrocarbon) fueled fires because these fires show a negligible spectral emission intensity near 4.4 microns. When the fire channel of the described system is configured to be responsive to IR radiation within a passband of 2.5 to 2.8 microns, the described system would be capable of detecting hydrogen fires as well as hydrocarbon fueled fires under certain circumstances. However, for hydrocarbon fueled fires, the preferred IR spectral region for fire detection is the bandpass centered 4.4. microns. In general, the magnitude or intensity of an emission in the 2.5 to 2.8 spectral region is less than that associated with an emission near 4.4 microns. Also, the magnitude of the emission in the 2.5 to 2.8 spectral region from non-sooty burning fuels (e.g., methane, ethane, propane) is much reduced as compared to that for sooty burning hydrocarbons (e.g., gasoline, jet fuels) which show blackbody continuum radiation in this range.
As indicated above, for the second sensor configuration it is possible for the described system to detect a hydrogen fueled fire. However, since the 2.5 to 2.8 micron passband is largely coincident with the atmospheric water vapor absorption band, the described system has a reduced ability to detect a hydrogen flame over a long atmospheric pathlength.
The fire channel detector, when configured to sense IR radiation in the 2.5 to 2.8 spectral region, will show increased response to solar radiation when such radiation is off-axis to the fire channel detector, a common situation. The off-axis shift or shift to shorter wavelength of the IR interference filter passband associated with this detector would cause the detector to become responsive to radiation having a wavelength shorter than 2.5 microns. Solar irradiance at the earth's surface becomes large for wavelengths shorter than 2.5 microns where atmospheric water vapor absorption of sunlight becomes small. An increase in response of the described system's fire channel due to solar radiation would decrease the overall system's sensitivity to respond to fires, since the differential between the system's reference channel and the fire channel output signals would become smaller.
In U.S. Pat. No. 4,357,534, a fire and explosion detection system is described. The particular application for the described system is the detection of hydrocarbon fuel fires in combat vehicles when the vehicles are struck by ammunition rounds. In this application, the system is configured so that it is responsive to hydrocarbon fires set off by an ammunition round or metal shards from the round but is not responsive to the exploding ammunition round or secondary non-hydrocarbon fires produced by the ammunition round striking the vehicle. This system configuration is used to actuate the vehicle's fire suppression system when an ammunition round is causing the vehicle's fuel to ignite.
In one embodiment, the described system consists of two channels, where the fire channel includes a narrow band detector and the reference channel a broad band detector. The narrow band detector is optically filtered to detect a relatively narrow band of IR radiation centered near 4.4 microns. The broad band detector is optically filtered to detect a relatively broad band of IR radiation also centered near 4.4 microns.
Alternatively, the system may be configured so that the broad band detector associated with the reference channel is further filtered by using a separate narrow band absorption filter in conjunction with a separate wide band transmission filter so that the broad band detector is made insensitive to a relatively narrow band of IR radiation centered near 4.4 microns and so the detector is sensitive to broad bands of IR radiation that are separated by this relatively narrow insensitive band. The narrow band corresponds to the bandwidth for the narrow band detector.
As indicated above, this system is not capable of simultaneously detecting fires fueled by hydrocarbons and/or non-hydrocarbons, rather the system is only configured to detect hydrocarbon fueled fires under certain conditions.
For both embodiments, the broad band detector associated with the reference channel is responsive to some IR radiation from both the water vapor fire emission band centered near 2.9 microns and the carbon dioxide fire emission band centered near 4.4 microns. This is because the cut-on and cut-off wavelengths associated with the reference channel passbands coincide with part of the wavelength region for the water vapor and carbon dioxide fire emission bands.
Also, because of the narrow width of the absorption filter associated with the alternate embodiment for the reference channel, off-axis shift of the passband in response to IR radiation from a fire located off-axis to the reference channel detector will cause increased response of the reference channel to a fire source. However, the off-axis shift of the narrow passband associated with the fire channel, will simultaneously cause the fire channel to become less responsive to fires located off-axis.
In general, it is desirous to have the reference channel non-responsive to the IR spectral regions where there are significant emissions from a fire. This assures that the signal in the reference channel will be significantly less than the signal from the fire channel detector, the detector used to sense the presence of fire generated radiation (i.e., the signal-to-noise ratio is maximized). It is also desirous not to have the fire channel become less responsive to fire generated radiation at the same time the reference channel is becoming more responsive.
In sum, U.S. Pat. Nos. 3,857,520 and 4,357,534 describe systems using multiple discrete sensors, sensing IR spectral regions which correspond to the IR radiation emitted by fires, and systems that are incapable of simultaneously sensing discrete IR spectral regions using a single detector to detect hydrocarbon or non-hydrocarbon fueled fires. Moreover, neither describes or teaches a system having a reference channel that simultaneously senses three IR spectral regions using a single detector and in particular a single IR optical filter. These systems are also affected by the effect of off-axis shift by the filters (i.e., the detector senses radiation having a shorter wavelength than that sensed when the incident radiation is on-axis).
In U.S. Pat. No. 5,162,658, a thermal detection arrangement is described that includes five active channels and one blind channel. The output of the blind channel is used to set the gain of an amplifier that amplifies the output signals from the active channel detectors and to make in process adjustments to the gain.
The system uses six separate thermal detectors with one thermal detector being allocated for each channel. Each thermal detector is optically filtered using a separate filtering device. The separate passbands associated with each of the active channels span the wavelength region of approximately 3.75 to 6.0 microns. As such, the system cannot detect non-hydrocarbon fueled fires since such fires do not show substantial radiant emission within this wavelength range.
Therefore, it is an object of the present invention to provide a fire detection system and method that can detect fires fueled by hydrocarbons, certain non-hydrocarbons or a combination thereof and which system has a low incidence of false alarms from non-fire radiation sources.
It is a further object of the present invention to provide a fire detection system which is non-responsive to non-fire radiation (e.g., the sun) or black body radiation sources which span a wide range of temperatures.
It is another object of the present invention to simultaneously sense IR radiation in a plurality of different and separate passbands using a system and methodology that is simple and which minimizes the costs for the associated optics, sensors and signal processing electronics.
It is yet a further object of the present invention to provide a fire detection system which is not significantly inhibited by the presence of oily films on the detector optics nor significantly inhibited by the presence of oil vapor or hydrocarbon smoke located in the optical path between the fire source and the detector.
It is yet another object of the present invention to provide a fire detection system and method that does not involve sensing ultraviolet radiation to detect the presence of a hydrocarbon and/or non-hydrocarbon fire.
It is still yet a further object of the present invention to detect fires fueled by hydrocarbons such as methane, ethane, propane, butane, alcohols (e.g., methanol, ethanol, propanol, butanol), diesel fuel, jet fuel and gasoline, and non-hydrocarbons such as hydrogen, hydrazine, silane, ammonia, and sodium azide, as well as any other fuels or substances which exhibit a strong emission in either or both the carbon dioxide and the water vapor emission bands centered near 4.4 microns and 2.9 microns respectively.